The present invention relates generally to medical devices, and more particularly to a method and apparatus for preventing rupture of blood vessels resulting from inflation of catheter balloons.
The flow directed balloon-tipped pulmonary artery catheter was introduced in 1970. While permitting easy bedside monitoring of cardiac function in critically ill patients, this device has resulted in certain complications, some benign and self-limiting, and others more significant. The latter include arterial and ventricular arrhythmias, balloon rupture, pulmonary infraction, bacteremia and intracardiac knotting. Rupture of the pulmonary artery is generally viewed as the most serious complication associated with the use of these balloon-tipped catheters. This is particularly a problem in patients over the age of sixty years, in whom it is believed that reduced vessel elasticity and increased fragility of tissues reduce the vessel rupturing pressure below the normal inflation pressures of the catheter balloons.
An explanation of a procedure employed with balloon-tipped catheters will result in a better appreciation of the magnitude of the problem. Use of inflatable catheters to internally block various body passages is well known in the art. Pulmonary artery catheters having inflatable balloons at their distal ends have been utilized in hundreds of thousands of procedures. The balloon, which in its uninflated state approximates the diameter of the catheter about which it is disposed, is inflated when desired by a gas syringe connected to the balloon channel or lumen of the catheter, which extends from outside the patient's body to the balloon.
To employ the catheter, a sheath is inserted in a suitable vein, such as the jugular vein, and the tip of the catheter is inserted through the sheath into the vein, until it exits the sheath, whereupon the balloon is inflated to act as a float responding to blood flow to assist in drawing the catheter through the vein until the balloon reaches the pulmonary artery. At this point, the balloon becomes wedged in a branch of the artery, and when lodged in this manner may be utilized to monitor the so-called "wedge" pressure of the artery via an oscilloscope connected to sensors in the catheter, all of which is known in the art.
After an initial wedge pressure reading is taken, the balloon is deflated and the catheter normally left in place in anticipation of subsequent readings. It is to this portion of the procedure that it is believed most problems with vessel rupture are attributable, for the deflation of the balloon at the distal end or "tip" of the catheter permits the now-smaller diameter tip to drift or migrate into a smaller portion or branch of the artery. If the migration phenomenon occurs, when next balloon inflation is attempted, the balloon may be positioned in a vessel of smaller diameter and/or greater fragility than intended. As a result, the normal inflation pressure of the balloon is exceeded due to the inability of the balloon to expand to its normal inflated diameter when injected with its specified volume of gas. Such balloon over-pressuring can result in catastrophic failure (rupture) of the vessel, generally resulting in rapid death of the patient.
Undue resistance to inflation of the balloon, which may indicate that the catheter tip is located in a small vessel, is a reason to terminate inflation attempts. However, such resistance, which may be transmitted to the practitioner through the "feel" of the plunger of the syringe being utilized for inflation, is subtle and difficult to interpret. Since balloons by various manufacturers provide differing inherent resistance to inflation, resistance to inflation attributable to a balloon being wedged in a small vessel may not be recognized by the feel of the syringe. This problem is aggravated with the small syringes typically used to inflate catheter balloons, since high pressures are indistinguishable from normal pressures to the clinician due to the small cross-sectional area of the plunger provided for pressure feedback from the catheter.
Pulmonary artery catheters of different manufacturers have widely differing balloon inflation pressures due to variations in thickness and latex composition of the balloons. When a balloon does inflate inside a vessel, it tends to "pop open" at a particular threshold pressure. The stiffer the balloon, the higher is the threshold pressure, with an attendant greater likelihood of vessel damage.
The concept of a pressure relief valve or regulator was believed to be useful in reducing the risk of vessel damage due to high balloon pressures because there is generally a 500 mm Hg difference between the normal opening pressure of the balloon and the minimum rupture pressure of the vessel. Use of such a valve at the balloon inflation port of a catheter has been suggested, as in U.S. Pat. No. 4,439,185 in the context of a liquid-inflated vascular dilating catheter, but the pop-open effect of the balloon negates the usefulness of such a device. Specifically, as a balloon pops open and thereby increases its volume, the internal pressure of the syringe/catheter/balloon system falls. Therefore, a relief valve set above the balloon pop-open pressure provides no protection after the balloon has opened; if set below the pop-open pressure, the pressure relief valve would prevent balloon inflation. Moreover, a pressure relief valve does not give a visual warning to the user that the catheter balloon has been inflated in a small vessel. Gas from the inflation syringe or other means escapes through the valve, thus preventing high inflation pressures, but without notice. If the catheter is subsequently inadvertently drawn back slightly to a larger diameter vessel, as often happens due to the motion of the heart or the patient, the balloon to vessel wall seal is lost and cannot be re-established in the larger vessel because the air or other gas required to do so has previously been vented through the pressure relief valve and cannot be returned into the system.
The addition of a "dead space" or pressure absorption zone between the syringe and the inflation port, such as has been suggested in U.S. Pat. No. 4,795,431, can be of use in limiting the maximum balloon pressure, as can the use of smaller than recommended gas volume for inflation, but both have proven unsatisfactory. It has been empirically determined that devices incorporating dead spaces or pressure absorption zones actually aggravates the vessel damage due to overpressure in certain diameter vessels. Use of a smaller than recommended gas volume results in an interior wedge blockage of vessels and an undesirable portion of the catheter tip remains exposed, enhancing the potential for vessel perforation, especially during catheter insertion. If the catheter tip resides in a small vessel, dangerously high balloon pressures can still result, especially if the vessel's internal diameter is close to the balloon's pop-open diameter.
Other attempts to identify over-pressurization of catheter balloons include external indicators or monitors associated with the catheter, such as indicator or "signal" balloons to provide the syringe operator with a tactile indication of the inflation pressure of the internal balloon, and pressure gauges. Both of these approaches, however, do not provide overpressure protection. At best they give a warning, but only after the patient has sustained significant injury from balloon inflation. Because of the pop-open effect of the balloon, a pressure gauge cannot indicate the actual pressure exerted on the vessel wall.